Since the finding that nitric oxide (hereinafter, nitric oxide may also be referred to as NO) acts as an essential component of a muscle-relaxing factor, the physiological function of NO has been elucidated, and utilization of NO as a neurotransmitter or an infection marker has been under consideration.
In particular, analysis of NO gas in exhaled air has been attracting attention as a marker for airway inflammation caused by, for example, asthma or an allergy, the patients of which are increasing in recent years. This type of analysis allows noninvasive diagnosis of disease without imposing a burden on patients. The concentration of NO gas in exhaled air of normal adults is 2 ppb to 20 ppb, but is known to increase by a factor of approximately three in cases of airway inflammation caused by, for example, asthma or an allergy. The concentration of NO gas in exhaled air of children is lower than that of normal adults. Therefore, in cases of children, it is necessary to measure the concentration of a trace amount of NO gas in their exhaled air. If a simple and compact measurement device capable of measuring a trace concentration of NO gas is realized, the device can be used in determining the degree of airway inflammation of a patient or in determining treatment plans for asthma such as a dosage of asthma medication.
Conventionally, measurement of NO gas in exhaled air is performed in the following manner: cause a reaction between a patient's exhaled air and ozone under a reduced pressure, thereby causing excitation of part of NO gas contained in the exhaled air; and detect light that is emitted when the excited state returns to the ground state. However, such a chemiluminescence method requires expensive peripheral devices such as an ozone generator, and the maintenance of such devices is laborious.
An inexpensive and compact NO gas measurement device that is excellent in terms of gas selectivity, capable of quick measurement, and has high sensitivity is necessary for allowing asthma patients to measure the concentration of NO gas in their exhaled air everyday at a hospital or at home for self asthma management.
In recent years, there has been disclosed a method in which cobalt tetrakis(5-sulfothienyl)porphyrin (hereinafter, referred to as Co{T(5-ST)P}) contained in a silica film fabricated through a sol-gel process is reacted with NO gas in a vacuum chamber, and NO coordinated to Co{T(5-ST)P} is detected by using an ultraviolet and visible spectrophotometer (see, for example, Non Patent Literature 1).
In this method, in order to achieve necessary NO gas selectivity, an amorphous silica film containing Co {T(S-ST)P} is formed in the following manner: slowly hydrolyze ethyl silicate for 24 hours in the presence of Co {T(S-ST)P}; apply a resultant solution onto a glass substrate; and dry the glass substrate. The film formed in this manner is used as a NO sensor. This method has succeeded in detecting 17 ppm of NO gas with a sensor temperature of 200° C.
Further, there has been disclosed a method in which a porous glass plate is immersed in a chloroform solution containing cobalt tetraphenylporphyrin (5,10,15,20-tetraphenyl-21H,23H-porphyrin cobalt (hereinafter, referred to as CoTPP)), and is then dried. In this manner, a NO sensor in which the porous glass plate has CoTPP supported thereon is formed (see, for example, Non Patent Literature 2). According to the disclosure, the sensor is placed in a reactor that has been vacuum-evacuated by an oil diffusion pump, and NO gas is detected by means of an infrared spectrophotometer or an ultraviolet and visible spectrophotometer.
Still further, there has been disclosed a method of optically detecting NO gas by using a NO sensor which is a sol-gel glass having cytochrome C included therein (see, for example, Patent Literature 1).
In the meantime, a report is made not about NO gas but about a gas detecting tape wherein silica gel particles impregnated with a reagent exhibiting a color to a gas to be detected are fixed to an adhesive layer (see, for example, Patent Literature 2).